1. Field of the Invention
This invention relates to a non-contact tonometer for blowing an air stream against the cornea of an eye to be examined to thereby deform the cornea, and optically detecting the deformation of the cornea to measure the eye pressure.
2. Related Background Art
In a tonometer of this type, whether the cornea-reflected light of a light beam for alignment detection travels in the direction of an air stream coaxial with the optic axis is photoelectrically detected. If the reflected light deviates from the optic axis or if the working distance deviates and the reflected light is not condensed on a photodetector, ejection of the air is stopped.
Referring to FIG. 1 of the accompanying drawings, E designates an eye to be examined, and Ec denotes the cornea thereof. A light beam for detecting the deformation of the shape of the cornea is projected as a parallel light beam from a light source 1 toward the cornea Ec of the eye E to be examined through a lens 2. The light reflected by the cornea is received through a lens 3 by a single photodetector 4 disposed on the optic axis and at the rearward focus position of the lens 3. On the other hand, a light beam for alignment detection is projected from a light source 5 onto the center of curvature of the cornea Ec via a lens 6, a beam splitter 7 and a lens 8, and when the cornea Ec is at a predetermined position, that is, when alignment is achieved, the light beam intactly returns along the same optical path, passes through the beam splitter 7 and a lens 9 and arrives at a photodetector 10. Also, compressed air provided by a cylinder mechanism comprising a cylinder 11 and a piston 12 is blown from a nozzle 13 disposed on the optic axis L toward the cornea Ec.
The direction of the air stream blown from the nozzle 13 toward the cornea Ec is coincident with the optic axis L, and when the cornea Ec is pressed flatly by this blown air, the light reception signal in the photodetector 4 becomes a maximum, and the eye pressure of the eye to be examined can be found from the air pressure at this point in time. The air pressure can be found from the output of a pressure sensor, not shown, which is provided in the cylinder 11.
Also, by the use of the cornea-reflected light of the light beam for alignment detection emitted from the light source 5, the alignment of the apparatus with the eye to be examined can be achieved because the output of the photodetector 10 rises when the cornea Ec is at a predetermined position. When there is misalignment, control means inhibits the measuring operation using the signal of the photodetector 10.
However, in the above-described example of the conventional art, the allowed alignment value is extremely narrow to ensure sufficient measurement accuracy, and much effort and time have been required for measurement and sufficient training has been necessary to accomplish efficient measurement.
The following factors are responsible for a measurement error caused by misalignment.
Firstly, part of the blown air stream escapes laterally and therefore, the amount of deformation of the cornea decreases and the eye pressure is measured to be too high. With regard to this, as shown in Japanese Laid-Open Patent Application No. 62-268524, the relation between misalignment and the apparent rise of the eye pressure is constant and this relation can be found in advance and corrected.
However, a measurement error occurs even for very slight misalignment. This occurs because the deformation of the cornea becomes asymmetrical due to the eccentricity of the blown air, and for example, the flatly pressed surface of the cornea inclines slightly from the perpendicular relative to the objective lens 8. In this case, the light beam for detection is condensed sideways from the light receiving element 4 disposed on the optic axis, and the predetermined deformation of the cornea cannot be accurately detected from the output of the light receiving element 4 and therefore a measurement error occurs.
Now, as another example of the prior art, there is one as shown in Japanese Laid-Open Patent Application No. 59-80228 wherein a sensor array is provided in the optical path of a cornea deformation detecting system and eye pressure is measured on the basis of the optical position of the cornea-reflected light on the sensor array. In this type of apparatus wherein such an optical position is found, the sensor array may basically be at any position in the direction of the optic axis relative to an eye to be examined, but in order that the optical position on the sensor array may be detected, the sensor array must be correctly aligned in a plane perpendicular to the optic axis relative to the eye to be examined. If measurement is effected in an inaccurate aligned state, the aforedescribed problem will likewise arise.
As a further example of the prior art, there is shown in Japanese Laid-Open Patent Application No. 60-83642 a tonometer in which the reflected light from the cornea of an eye to be examined is caused to enter three array sensors arranged in three meridian directions to thereby accomplish alignment, but this also is merely a tonometer of the type in which alignment is accurately effected before the measurement of eye pressure.